(Invited) Nanoporous Membranes By Self-Assembly of “Hairy” Nanoparticles

Nanoporous membranes attract increasing attention due to their applications in molecular separations, biosensing, drug-delivery and catalysis. Most currently used membranes are formed by introducing pores into the bulk materials in a variety of irreversible processes. However, nanoporous membranes can be also prepared in a simple self-assembly process from nanoparticle building blocks, since particle solids contain interstitial voids which can function as interconnected three-dimensional nanopores. Membranes formed from nanoparticles using non-covalent interactions possess an additional advantage of reversible assembly which could prove useful for fabrication, processing, cleaning, reusing and would possess facile control over the membrane porosity and nanopore surface properties. This talk will describe the preparation and characterization of durable, reversible nanoporous membranes with controlled thickness, area and pore size by self-assembly of silica nanoparticles grafted with polymer brushes ("hairy" nanoparticles, HNPs). The nanoparticles used to form the membranes are prepared by first modifying their surface with 2-bromoisobutyrylbromide (polymerization initiator) followed by surface-initiated atom transfer radical polymerization of a variety of methacrylate monomers. Nanoporous membranes are formed by casting HNPs from their suspensions in suitable solvents. Three types of HNP membranes will be discussed: membranes made of HNPs grafted with polymer brushes carrying acidic and basic groups, membranes in which the grafted polymer brushes have neutral groups, and membranes grafted with negatively charged polymer brushes. Depending on the HNP type the membranes are stable in organic solvents but disassemble in water, or stable in water but disassemble in organic solvents. All types of HNP membranes are size-selective. When negatively charged groups are introduced to the side chains of the polymer brushes, the corresponding membranes a capable charge rejection. This talk will describe the different mechanisms of transport selectivity in nanoporous HNP membranes as well as the nanoparticle interactions in these materials that lead to their formation and stability.